The human heart is divided into four chambers. These include the right atrium, the right ventricle, the left atrium, and the left ventricle. The right atrium and right ventricle are divided from the left atrium and left ventricle by a muscular wall called the septum. The atrial septum is the wall separating the atria and the ventricular septum is the wall separating the ventricles.
Both the right and left atrium have a pouchlike structure attached to the atrium, which is called the right atrial appendage and the left atrial appendage (LAA). The LAA is the remnant of the original embryonic left atrium that develops during the third week of gestation. The LAA is a long, tubular, hooked structure which is usually crenellated and has a narrow junction with the venous component of the atrium.
In adults, there are no known uses for the left and right atrial appendages, like the appendix in the intestines. In the normal heart, the appendages contract, along with the rest of the atrial muscle and the blood moves in and out of the atrial appendages. In atrial fibrillation, there is a lack of synchronous or uniform contraction of the atrium muscle. Thus, the blood in the appendages remains dormant and does not move. In the right atrium, this has not been a health problem. In the LAA however, dormant blood can cause health problems. When the blood remains dormant in the LAA, thrombus has a tendency to form. In some patients, thrombus in the LAA can leave and travel within the cardiovascular system. In some instances, thrombus can travel to the brain and result in a stroke.